초록
<P><B>Background</B></P><P>Lactic acid is the building block of poly-lactic acid (PLA), a biopolymer that could be set to replace petroleum-based plastics. To make lactic acid production cost-effective, the production process should be carried out at low pH, in low-nutrient media, and with a low-cost carbon source. Yeasts have been engineered to produce high levels of lactic acid at low pH from glucose but not from carbohydrate polymers (e.g. cellulose, hemicellulose, starch). <I>Aspergilli</I> are versatile microbial cell factories able to naturally produce large amounts of organic acids at low pH and to metabolize cheap abundant carbon sources such as plant biomass. However, they have never been used for lactic acid production.</P><P><B>Results</B></P><P>To investigate the feasibility of lactic acid production with <I>Aspergillus</I>, the NAD-dependent lactate dehydrogenase (LDH) responsible for lactic acid production by <I>Rhizopus oryzae</I> was produced in <I>Aspergillus brasiliensis</I> BRFM103. Among transformants, the best lactic acid producer, <I>A. brasiliensis</I> BRFM1877, integrated 6 <I>ldhA</I> gene copies, and intracellular LDH activity was 9.2 × 10<SUP>−2</SUP> U/mg. At a final pH of 1.6, lactic acid titer reached 13.1 g/L (conversion yield: 26%, w/w) at 138 h in glucose-ammonium medium. This extreme pH drop was subsequently prevented by switching nitrogen source from ammonium sulfate to Na-nitrate, leading to a final pH of 3 and a lactic acid titer of 17.7 g/L (conversion yield: 47%, w/w) at 90 h of culture. Final titer was further improved to 32.2 g/L of lactic acid (conversion yield: 44%, w/w) by adding 20 g/L glucose to the culture medium at 96 h. This strain was ultimately able to produce lactic acid from xylose, arabinose, starch and xylan.</P><P><B>Conclusion</B></P><P>We obtained the first <I>Aspergillus</I> strains able to produce large amounts of lactic acid by inserting recombinant <I>ldhA</I> genes from <I>R. oryzae</I> into a wild-type <I>A. brasiliensis</I> strain. pH regulation failed to significantly increase lactic acid production, but switching nitrogen source and changing culture feed enabled a 1.8-fold increase in conversion yields. The strain produced lactic acid from plant biomass. Our findings make <I>A. brasiliensis</I> a strong contender microorganism for low-pH acid production from various complex substrates, especially hemicellulose.</P>